With the emergence of real-time magnetic resonance imaging (MRI) techniques, the use of MRI has expanded from static diagnostic imaging to include the potential to guide a variety of interventions. Many percutaneous cardiovascular procedures (i.e., interventions performed with a catheter inserted into the vasculature) may benefit from guidance where MRI's soft tissue contrast may be exploited. One example is the traversing of chronic total occlusions in coronary and peripheral vessels. The presence of chronic total occlusions is the leading reason for selection of bypass surgery over less invasive interventions. Despite the benefits of percutaneous treatment, clinicians are often unable to traverse occlusions with catheter-based devices due to the inadequate imaging capabilities of X-ray fluoroscopy that is typically used to image such treatment.
Reference is now made to FIG. 1. Typically, during percutaneous interventions two pieces of equipment are inserted into the vasculature 10. The first is a catheter 12 that may be a long thin hollow tube. The second is a guidewire 14, which is typically thin flexible wire that may travel through the lumen of the catheter 12. FIG. 1 shows a schematic diagram illustrating the use of a conventional guidewire 14 and catheter 12 in the vasculature 10 of a patient. Typically, the guidewire 14 is extended from the catheter tip, and because the guidewire 14 is usually very flexible, it is the first device to be manoeuvred through the vasculature 10. The catheter 12 is advanced over top of the guidewire 14 to provide mechanical support, and when pushed, the catheter 12 follows the path of the guidewire 14.
Several MRI-guided guidewire tracking and visualization techniques have been proposed, which may be classified into two groups. The first group may be referred to as “passive techniques” where the device is made visible through the use of signal voids, susceptibility artifacts, or off-resonance signals (e.g., those discussed in References 1-4). These techniques typically are limiting in that the device must lie within the MR imaging plane in order to be viewed.
The second group may be referred to as “active techniques”. Active techniques rely on an acquisition of the magnetic resonance (MR) signal from small micro-coils or wires located on the device in order to determine device position (e.g., as discussed in References 5 and 6). Active visualization techniques typically do not suffer from the same limitations as passive techniques due to the fact that the signal used for device localization is acquired independently from that used for anatomical imaging. This enables the device to be located even when it lies outside the current imaging plane. Moreover, because the signal from the device is a separate signal, it may be colour-overlaid on anatomical images to create a “positive contrast” that may be easy to identify and put in an anatomical context. However, active visualization of the guidewire may be challenging in that many of the techniques developed for catheters and endoscopes (e.g., the use of micro-coils) are difficult to translate to guidewires due to the limited thickness of guidewires. Guidewires are thin wires with a typical diameter of less than 0.035 inches, whereas catheters and endoscopes may have a much larger diameter which allow for accommodation of components necessary for this visualization.
Some current active guidewire designs consist of a loopless antenna that is formed on the end of a coaxial cable (e.g., Reference 7). This design includes two limitations. The first is that the active wires typically require significant internal structure. A result of this is that the mechanical properties of the guidewire do not resemble that of a conventional bare wire, which may affect its manoeuvrability in the vasculature. Further, active guidewires may be considered to be unsafe because resonant currents may develop on the outside conductor of the thin coaxial cable used to carry the MR signal from the loopless antenna to the input of the MR scanner (e.g., as discussed in References 8-11). These resonant currents may create intense localized heating of tissues located at the ends of the active guidewire. The same safety concern exists regarding the use of traditional non-active guidewires in the MR scanner.
Reference is now made to FIG. 2. A design for a MR-compatible guidewire 20 has been proposed that consists of a short non-resonant length of nitinol connected to a non-conducting fibreglass rod (e.g., as discussed in References 12 and 13). The non-conductive length may be made of any non-conductive material, including fibreglass, graphite, carbon fibre, or a polymer. FIG. 2 illustrates a schematic diagram of such a guidewire 20. In this schematic, the guidewire 20 has a non-resonant conductive length 22 (e.g., approximately 10 cm) of nitinol at the distal end attached to a non-conducting length 24 (e.g., a fibreglass rod) that forms the remaining length of the guidewire 20. The length of nitinol 22 is non-resonant and thus large currents are unable to develop in the guidewire 20. Such a guidewire 20 is therefore not susceptible to the heating concerns discussed above. Visualization of the guidewire 20 is done passively by doping the conductive length 22 and non-conductive length 24 with small iron particles. This creates a susceptibility artifact that may be seen on MR images. However, this method suffers from the same limitations as other passive visualization methods, including the limitation that the guidewire 20 may be visualized only when it is in the imaging plane.